Digital Tests Move Beyond Logic

When engineers must test a digital system, board, or integrated circuit, their test equipment should do more than supply and monitor standard logic levels. The digital-test landscape requires attention to signal integrity, differential-signaling techniques, and test software.

Perhaps the biggest test issue centers on ensuring digital inputs work properly. First, proper testing requires accurate and stable power sources under control of test software. Suppliers of ICs used in laptop computers and cell phones, for example, offer ASICs that operate at 3.3V. But because computers and phones rely on battery power, test engineers must ensure these ASICs operate properly within a range of battery voltages. Programmable power sources use test software to vary supply voltages while other test equipment apply stimuli to an ASIC and monitor test results.

Second, test equipment also must vary the levels of the digital signals applied to a device under test (DUT) to match the changing supply voltage mentioned above. By varying power and logic voltages in concert, software can determine how well an ASIC will work with a range of real-world conditions. In essence, the test engineers determine an operational "window" for a DUT.

Third, signal sources also should let test engineers independently change logic voltages for each input. Doing so will reveal any sensitivities to small changes in the logic levels at DUT's inputs. A tester for ASICs may require several hundred such programmable sources under control of a test program.

What's the Difference?

Newer digital systems may use low-voltage differential signaling (LVDS) devices to communicate. These devices use a bipolar current that produces a small voltage across a resistor at an LVDS receiver's inputs. The low currents and voltages involved decrease switching times so transmission rates can reach 1 Gbit/sec. And LVDS devices aren't sensitive to specific input voltages or logic voltage levels.

Unfortunately, test equipment generally provides single-ended logic signals, not the bipolar current signals generated by LVDS transmitters. So, engineers who plan to test LVDS circuits need programmable current sources that rapidly switch polarity and operate out of phase with no timing skew. Most bench instruments can't easily handle this requirement, although several PXI-based instrument cards can. Keep in mind that many signal sources and measuring instruments act like 50-ohm loads, so calculations of test conditions must account for these impedances. Most circuits today may include only a few dozen LVDS inputs, but expect the use of LVDS circuits to increase in systems that must communicate rapidly.

Programmable digital outputs from a tester can serve other purposes beyond generating variable logic and LVDS signals. A programmable voltage source under software control can quickly test protection diodes on inputs and outputs. These diodes protect an IC by shunting to ground any harmful electrostatic discharge (ESD) currents. Testing these diodes involves increasing the input voltage at an ASIC's input while simultaneously measuring the input voltage and watching for a plateau that indicates diode conduction.

Proper testing of digital devices often relies on "vectors," or patterns of 1's and 0's that stimulate a DUT. Some engineers may require only an Excel spreadsheet to create vectors for a simple test. On the other hand, high-end circuit-simulation programs include automatic test-pattern generators (ATPGs) that create test vectors to exercise a digital DUT. In between Excel and simulators, engineers will find digital-waveform editing software that creates test patterns through use of a graphical interface. Simply draw a signal and the software creates an equivalent test vector.

JTAG, You're It

No digital test strategy should go without some form of boundary-scan or JTAG testing. (JTAG stands for the Joint Test Action Group that established the boundary-scan standard.) This technique uses circuits built into standard and custom ICs to test for the proper connections between ICs on printed-circuit boards (PCBs). A PCB that implements boundary-scan testing furnishes a four- or five-signal connection—a JTAG port—that links it to a host computer or instrument system. The IEEE 1149.1 standard defines boundaryscan signals and protocols.

But designers can't leave design-for-test capabilities such as JTAG to the last minute. From the start, they must implement circuits to disable clocks or to temporarily connect inputs on some devices to logic-1 or logic-0 for testing. PCB design must accommodate signal paths for boundary-scan connections and must include space for a small JTAG connector. Companies that supply JTAG hardware and software provide design guidelines that can help both circuit and board designers implement boundary-scan. As with any test process, using boundary scan requires planning.

Many ASIC and microprocessor vendors also use an on-chip JTAG port as a gateway for built-in self-test (BIST) capabilities. Manufacturers of devices that include FLASH memory often allow for programming via a JTAG port.

The capability to program FLASH memory through a JTAG port can permit programming on a production line as well as firmware upgrades in the field.

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